It is known that organic and polymeric materials with large delocalised .pi.-electron systems may have optically non-linear coefficients which are larger than those of inorganic materials.
The properties of organic and polymeric materials also can be readily varied so that valuable secondary properties, such as mechanical and chemical stability, optical absorption, etc., can be set without negatively affecting non-linearity.
Thin films of organic or polymeric materials having considerable non-linearity of the second order have a great potential for use in the areas of optical communication, laser technology, electro-optics, and related fields.
It is particularly significant that the non-linearity of these materials is due to the ability of the .pi.-electron system to be polarizable and not due to a shift or a reorientation of atoms or molecules. Components having ultra-short response times, therefore, can be realised with these materials.
This field has been the subject of intensive research for many years and is documented, for example, in the following publications:
ACS Symposium Series 233, Washington D.C. 1983;
Proceedings of SPIE Vol. 682 1986 Molecular and Polymeric Optoelectronic Materials: Fundamentals and Applications;
Proceedings of SPIE Vol. 824 1987 Advances in Nonlinear Polymers and Inorganic Crystals, Liquid Crystals, and Laser Media;
Proceedings of SPIE Vol. 1105 1989 Materials for Optical Switches, Isolators, and Limiters;
Proceedings of SPIE Vol. 1147 1989 Nonlinear Optical Properties of Organic Materials 2;
NATO ASI Series E: Applied Sciences Vol. 162 1989 Nonlinear Optical Effects in Organic Polymers;
Springer Proceedings in Physics 36 1989 Nonlinear Optics or Organics and Semiconductors.
Particularly important for these applications are side-chain polymers and liquid-crystalline side-chain polymers which are doped with non-linear optical (nlo) chromophores or having nlo groups as side chains. For the purpose of this description, the term side-chain polymers also includes co-polymers and homopolymers. It is known that suitably selected representatives of this kind of materials can be given a dipolar orientation by heating above its glass temperature in a high electric field and retaining this non-centro-symmetrical order by cooling below the glass temperature Tg under the applied electric field. Frozen poled glasses of this kind represent nlo materials with high .chi..sup.(2) susceptibility (K.D. Singer et al. ,Appl. Phys. Let. 49, 1986).
A basic obstacle to the technical use of these materials is the inadequate long-term stability of the dipolar order. It has been found that relaxation occurs even below the glass temperature of the polymer system and destroys the previously imposed non-centro-symmetrical structure.
There are various strategies for increasing the stability of the dipolar order by modification of the polymer. Since the relaxation processes slow down with increasing distance from the glass temperature, one method is to select polymers having the highest possible glass temperature. Another method is to use cross-linked or branched polymer networks instead of linear polymers.
Much better long-term and temperature-stable nlo-active polymers have been obtained by using nlo chromophores with several (2-4) reactive substituents to act as crosslinking units in an epoxide system (M. Eich J. Appl. Phys. 66, 3241, 1989). In these epoxides, the cross-linking reaction takes place under the influence of an electric field and results in a partially dipolar-oriented network. This orientation is retained after the field has been switched off, because the dipolar order is chemically fixed.
The simple and precise patterning of nlo-active polymer networks is of fundamental importance for its technical use. Since, in the prior art processes, the cross-linking was initiated thermally, it encompasses the entire coating and cannot be restricted to selected partial areas.
Thus the generation of a geometrical pattern is only possible by the use of structured poling electrodes. However, this is accompanied by a number of serious disadvantages. Unavoidable electric stray fields at the edges of the electrodes cause the boundaries of the poled zones to become blurred. This is unacceptable for many applications, for example, strip waveguides, periodic structures of poled and unpoled regions etc.. Also, for most applications the electrode coatings have to be tediously removed again in later additional steps.